IGBTs (insulated gate bipolar transistors) are widely used as power semiconductor elements with a breakdown voltage of approximately 300 V or more. Such IGBTs are often used as switching elements in power supply circuits and inverter circuits. In this case, a free-wheeling diode connected in reverse parallel to the IGBT is needed to pass a sustaining current due to the inductor in these circuits or in the loads connected to these circuits. There is demand for downsizing of power semiconductor devices. Furthermore, there is demand for a power semiconductor device including a free-wheeling diode and an IGBT element in the same chip.
In a power semiconductor device including an IGBT and a free-wheeling diode in the same chip, in order to prevent extension of the depletion layer to the dicing line of the chip, an n-type channel stopper layer is provided at the surface of the terminal portion of the IGBT chip. This serves as a cathode layer electrically connected to the collector electrode of the IGBT. Furthermore, a p-type diffusion layer formed at the outer periphery of the IGBT element region serves as an anode layer connected to the emitter electrode of the IGBT element. A free-wheeling diode, with this p-type diffusion layer serving as an anode layer and the channel stopper layer serving as a cathode layer, is integrally formed so as to be connected in reverse parallel to the IGBT region.
In such a power semiconductor device, when a positive voltage relative to the collector electrode of the IGBT is applied to the emitter electrode, a current flows in the free-wheeling diode along a path composed of the emitter electrode, p-type semiconductor layer, n-type epitaxial layer, channel stopper layer, and collector electrode. However, in this free-wheeling diode, the current concentrates near the surface of the n-type epitaxial layer, and hence the on-resistance of the free-wheeling diode is relatively high.